Why the world seems smooth to eyes?

Washington: Our eyes are constantly taking in new sights, jumping from one image to another. However, even as they make these lightening shifts, the world always appears smooth and whole. Now, boffins have just how this phenomenon occurs.

Researchers at the University of Pittsburgh and the National Eye Institute (NEI) for the first time have found just how the brain's visual system "knows" where the eyes are about to move and is able to adjust for that movement.

The lead researchers on the study were Marc Sommer, assistant professor of neuroscience at Pitt and Robert Wurtz, senior investigator at NEI, one of the National Institutes of Health.

Sommer and Wurtz, working on the 1950 hypothesises of Nobel laureate Roger Sperry that when the brain commands the eyes to move, it also sends a corollary discharge, or internal copy, of that command to the visual system, showed in an earlier 2002 Science paper that a pathway from brainstem to frontal cortex conveys a corollary discharge signal in the brains of monkeys.

They suggested that this pathway might cause visual neurons of the cortex to suddenly shift their receptive field - their window on the world - just before a saccade or little jumps.

Such neurons with shifting receptive fields had been discovered by Pitt Professor of Neuroscience Carol Colby and colleagues in 1992.

In their current paper, the boffins completed the circuit, and have shown that receptive fields in cortex are shifted because of the corollary discharge from the brainstem.

To do this, they exploited the fact that the signals are relayed via the thalamus, a crucial intermediary. By knocking out the relay neurons, they interrupted the pathway. They found that receptive field shifts were curtailed by more than half.

A similar circuit is likely to exist in human brains, the researchers say.

"This is a classic problem in neuroscience. People have been searching for a circuit to accomplish this stability for the last 50 years, and we think we've made good progress with this study," Marc Sommer said.

The study will be published in the November 16 print edition of Nature.